Anti-Angiogenesis Drug Discovery and Development: Volume 5 E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Fachliteratur
- Serie: Anti-Angiogenesis Drug Discovery and Development
- Sprache: Englisch
The inhibition of angiogenesis is an effective mechanism of slowing down tumor growth and malignancies. The process of induction or pro-angiogenesis is highly desirable for the treatment of cardiovascular diseases, wound healing disorders, etc. Efforts to understand the molecular basis, both for inhibition and induction, have yielded fascinating results.
Anti-angiogenesis Drug Discovery and Development provides an excellent compilation of well-written reviews on various aspects of the anti-angiogenesis process. These reviews have been contributed by leading practitioners in drug discovery science and highlight the major developments in this exciting field in the last two decades. The feast of these reader-friendly reviews on topics of great scientific importance – many of which are considered significant medical breakthroughs, makes this series excellent reading both for the novice as well as for expert medicinal chemists and clinicians.
The fifth volume brings together reviews on the following topics:
- Targeted therapy for tumor vasculature
- Anti-angiogenic therapy for breast and prostate cancers (including information updates on clinical trials)
- Microbe-based and other novel antiangiogenesis therapies, including chromene-based agents
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Seitenzahl: 370
Veröffentlichungsjahr: 2020
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PREFACE
Angiogenesis, formation of new blood vessels, is one of the most vigorously studied processes due to its central place in normal physiology and in the on-set of diseases. Imbalance in tightly regulated angiogenesis leads to complex chronic disorders, especially solid tumors and other malignancies. A complex cascade of angiogenic signaling molecules, including vascular endothelial growth factors (VEGF), is involved that has attracted immense scientific interest as possible drug targets. Several inhibitors of angiogenesis have been developed and extensive preclinical and clinical studies are underway.
The book series “Anti-angiogenesis Drug Discovery and Development” covers a broad range of topics focusing on the development of anti-angiogenic agents and their mechanisms of actions. Volume 5 has five comprehensive reviews, contributed by leading experts in these fields. These reviews broadly cover various important drug targets, and new classes of anti-angiogenic therapies for the prevention or treatment of cancers.
The review by Mabeta et al focuses on complex molecular mechanisms of angiogenesis, and their role as drug targets. Therapeutic approaches that target multiple pathways and components of the microenvironment as well as those normalize the vasculature, are explored. In addition, applications of noninvasive imaging to monitor the effects of AIs on tumor vessels is discussed. The review of Khan et al. focuses on various types of antiangiogenic agents, biochemical, chemical, and recombinant, in clinical trials or those recently approved against breast and prostate cancers and their respective mechanisms of action.
Microorganisms have been rich sources of biologically active molecules since the discovery of penicillin. Khurshid et al have reviewed the recent literature on innovative approaches involving microbe-based anti-angiogenic anti-cancer therapeutics. These include DNA vaccination, bactofection, alternative gene therapy and RNA interferences, as well as genetically modified bacteria as antiangiogenic agents. Soleimanjahi and Ala Habibian have contributed a comprehensive review on various classes of antiangiogenic agents which target powerful regulators that play a crucial role in cell proliferation and tumor microenvironment. These agents include VEGF inhibition, mesenchymal stem cells (MSCs) as vehicles for antiangiogenic agents, use of oncolytic viruses which selectively replicate in tumor cells, etc. Last but not the least, the review of Costa et al covers recent progress on coumarin- and chromene-based anticancer agents. Chromenes and coumarins are naturally occurring polyphenolic compounds, which are known to regulate the expressions of VEGF, matric metalloproteinases (MMPs), and receptor tyrosine kinases (RTKs) signaling pathways. Many of them are in various stages of development.
We would to express our gratitude to all the authors of above cited review articles for their excellent scholarly works in this dynamic and exciting field of biomedical and pharmaceutical research. The efforts of the excellent team of Bentham Science Publishers for the timely production of the 5th volume are greatly appreciated particularly the efforts of Ms. Mariam Mehdi (Assistant Manager Publications), and Mr. Mahmood Alam (Director Publications). We hope that the efforts of the contributing authors, and the production team will help readers in gaining a better understanding of this important area of biomedical research.
LIST OF CONTRIBUTORS
Therapeutic Targeting of the Tumor Vasculature: Past, Present and Future
Peace Mabeta1,*,Mike Sathekge2,Vanessa Steenkamp3Abstract
Tumor progression relies on a constant supply of oxygen and nutrients. Angiogenesis, the formation of neovessel from existing microvessels, is a prerequisite for the growth of many tumors. Significant advances have been made in delineating the interplay between pro- and anti-angiogenic factors that foster an environment that promotes the angiogenic phenotype in tumors. Of these angiogenic regulators, vascular endothelial growth factor-A (VEGF-A) and its cognate receptor, vascular endothelial growth factor receptor-2 (VEGFR-2) have been the most studied.
Various angiogenesis inhibitors (AIs) that target VEGF-A and VEGFR-2 have been developed for use as monotherapy or as part of combination therapies with standard chemotherapy. However, these AIs have thus far produced modest results, in part owing to compensatory pathways that have led to disease refractory.
To overcome refractory to disease, normalization of the tumor vasculature and broadening of the scope of therapeutic targeting are necessary. Furthermore, predictive biomarkers can enhance efficacy by enabling the early detection of resistance as well as the determination of clinical benefit. Herein, the therapeutic approaches that target multiple pathways and components of the tumor microenvironment, as well as those that normalize the vasculature are explored. In addition, the future application of non-invasive imaging to monitor the effects of AIs on the tumor vessels is discussed.
INTRODUCTION
The functions of the blood vasculature include the transport of nutrients and gasses, transcapillary filtration, vascular tone, hemostasis, and hormone traffic-king [1]. Endothelial cells provide a non-thrombogenic environment which facilitates the transit of plasma and cellular constituents of blood throughout the vasculature [1, 2]. The functions of blood vessels are mainly effected through various ligands including growth factors and their receptors, cytokines, as well as transcription factors such as hypoxia inducible factor-1α (HIF-1α) [2, 3]. Some of these molecules also modulate vascular homeostasis by regulating the process of angiogenesis.
Angiogenesis is the formation of blood vessels from an existing microvasculature [4, 5]. During embryonic development, angiogenesis is necessary for the remodeling of the primordial vasculature, while in postnatal development, the process of angiogenesis is important for tissue growth [5]. However, in the adult, the endothelium is in a relatively quiescent state, which is maintained by a balance between anti-angiogenic and pro-angiogenic factors [6]. Under such conditions, angiogenesis is observed only in a few instances such as during wound healing. In the female, vascular homeostasis is also periodically interrupted during the reproductive cycle [7].
The process of angiogenesis also occurs during pathological conditions such as cancer [2, 8]. Indeed, sustained angiogenesis is a hallmark of various malignancies [9, 10].
ANGIOGENESIS IN TUMOR PROGRESSION
For tumors to grow beyond 1-2mm in diameter, they need to elaborate a vascular supply [2]. Angiogenesis enables tumor neovascularization, thus providing a route for nutrient delivery [2, 8]. The steps involved in angiogenesis include the disruption of the basement membrane and remodeling of the extracellular matrix (ECM) [8].
In addition, endothelial cells (ECs) migrate and proliferate in the direction of the stimulus, and coalesce to form capillary channels through which blood flow can be established [6, 8]. The maturation of the newly formed is the recruitment of supporting cells such as pericytes [4, 9]. Unlike blood vessels formed through physiological angiogenesis, the tumor vessels are structurally and functionally abnormal.
The Tumor Vasculature
Tumor blood vessels are chaotic and lack the normal architecture observed in normal vessels. These blood vessels are also tortuous, and they have few pericytes, making them immature and unstable [2, 6]. Furthermore, due to the loss of VE-Cadherin, cell-cell contacts are compromised, resulting in a loss of vessel integrity [6].
Some of the vessels are susceptible to collapse, thus leading to erratic blood flow and poor perfusion [2]. This in turn hampers proper delivery of drugs. Also, the vessels are leaky, which supports tumor cell extravazation into the circulation [5, 6]. It is for this reason that vascular normalization has been recognized as important for effective tumor antiangiogenesis therapies.
The trigger for tumor angiogenesis is a local imbalance between pro-angiogenic factors and anti-angiogenic factors, which is tilted towards angiogenesis stimulators [2, 6]. In response to hypoxia, tumor cells secrete proteins that stimulate angiogenesis (Fig. 1). Of the secreted proteins, the vascular endothelial growth factor-A (VEGF-A) is the best characterized [11].
Fig. (1)) Schematic representation of the induction of angiogenesis by angiogenic growth factors secreted by tumor cells. Proangiogenic factors such as VEGF stimulate the vasculature to sprout new vessels. The newly formed vessels enable further tumor growth. Angiogenesis inhibitors limit tumor neovascularization and sometimes normalize the tumor vasculature. AIs – angiogenesis inhibitors- vascular endothelial growth factor (VEGF),- basic fibroblast growth factor (bFGF), - platelet derived growth factor (PDGF)Importance of Vascular Endothelial Growth Factor Signaling in Cancer
In humans, the vascular endothelial growth factor (VEGF) family consists of VEGF -A, -B, -C, -D, and placental growth factor (PlGF) [12]. The most studied angiogenic growth factor in this family is VEGF-A (referred to herein as VEGF). It is an important mediator of angiogenesis both in physiological and pathological settings [12]. The principal receptors for VEGF are VEGFR-1 and VEGFR-2, although VEGFR-2 presents greater signaling activity [13]. As such, the mitogenic actions of VEGF in endothelial cells are mediated mainly via VEGFR-2 [12, 13]. In addition, VEGFR-2 plays a key role in modulating cell migration and vascular permeability in response to VEGF, whereas VEGFR-1 has a weak or undetectable response [12].
Upon binding of VEGF to VEGFR-2, phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3k) is recruited to the internal side of the cell membrane and activated through phosphorylation (Fig. 2) [14, 15]. Activated PI3k phosphorylates phosphatidylinositol 4,5-bisphosphate (PIP2) to form phosphatidylinositol 3,4,5-trisphosphate (PIP3) [14].
Fig. (2)) A schematic diagram showing the activation of PI3K/PKB signaling which occurs following the binding of VEGF to VEGFR-2. The pathway promotes angiogenesis.Phosphatidylinositol 3,4,5-trisphosphate initiates a cascade of events that lead to the phosphorylation of 3-phosphoinositide-dependent protein kinase-1 and -2 (PDK1/2), which in turn activate protein kinase B (PKB), a key downstream effector of PI3k, ultimately leading to angiogenesis (Fig. 2) [14, 15].
Investigations have shown unequivocally that angiogenesis promotes tumor progression and in some cancers, metastasis. Accordingly, the targeting of the tumor vasculature has become an important approach in anti-cancer therapy [8, 10, 11]. In particular, the development of drugs that target vascular endothelial growth factor signaling has received considerable attention over the past two decades.
Studies on human and murine tumors have shown that VEGFR-2 is upregulated in tumors [8, 16]. Also, high levels of VEGF have also been measured in blood and urine samples of patients with different neoplasms [17-19]. Additionally, in some cancers, high levels of VEGF correlate with a poor prognosis [19]. The clinical development of angiogenesis inhibitors (AIs) has also focused on the cognate receptor for vascular endothelial growth factor, namely, VEGFR-2 [18, 19].
Therapeutic Targeting of Vascular Endothelial Growth Factor
The discovery that VEGF is over-expressed in many neoplasms, and that it is a key mediator of tumor angiogenesis, made it an important target in anti-cancer therapy. Preclinical studies in xenograft models of various cancers found that the inhibition of VEGF resulted in a decrease in microvascular density (MVD) and in the suppression of tumor growth [20].
The inhibition of VEGF with a neutralizing antibody led to reduced metastasis in preclinical models of colorectal cancer [20]. Based on these observations, the targeting of VEGF was further tested in patients.
It is worth noting that the clinical testing of a VEGF targeting antibody was preceded by the development of TNP-470, an analogue of Fumagillin. TNP-470 was one of the first drugs to undergo clinical testing for angiogenesis inhibition in cancer patients [21]. Unfortunately negative results such as a short half-life and severe side-effects hampered its further clinical development. Nonetheless, efforts to develop anti-angiogenic drugs continued and by 2004, Ferrara and colleagues had designed a humanized VEGF antibody (Ab) to target the tumor vasculature [16].
Although the neutralization of VEGF with a VEGF antibody was successful in preclinical studies, the humanized Ab was much weaker than the murine Ab in terms of binding affinity to the growth factor [22]. The humanized Ab was thus further engineered by replacing 7 residues in the variable heavy domain [22]. The newly engineered human VEGF monoclonal antibody, which was named bevacizumab, underwent testing in patients with diseases characterized by excessive angiogenesis such as macular degeneration and cancer.
In initial studies, bevacizumab did not improve the overall survival of breast cancer patients when compared to chemotherapy [22]. However, in further clinical studies the drug was shown to improve both overall survival (OS) and progression free survival (PFS) in metastatic colorectal cancer as either first line or second line treatment [16, 23]. In 2003 a positive outcome was obtained in phase III trials when bevacizumab was used in combination with chemotherapy. The study showed that patients receiving a combination of bevacizumab plus chemotherapy had a 50% increased chance of survival compared to patients who received chemotherapy alone [23].
In February 2004 bevacizumab became the first AI to be granted approval by the United States Food and Drug Administration (FDA) for the treatment of metastatic colorectal cancer [16].
Bevacizumab has had response rates of up to 10% as monotherapy, with higher response rates of 55% being observed in glioblastoma multiforme [24]. Promising results were also observed in lung, ovarian, endometrial, mesothelioma and cervical cancers as indicated by increased progression-free survival [22]. Bevacizumab is approved for metastatic colorectal cancer, non-squamous non-small cell lung cancer, glioblastoma and metastatic renal cell carcinoma [23]. Bevacizumab, in combination with interferon alpha (IFNα), has become standard therapy for metastatic renal cell carcinoma (mRCC) [25].
However, the observation in cancer patients treated with bevacizumab was that following the discontinuation of therapy, the vasculature is re-established [26]. This necessitated prolonged use of the drug in order to improve therapeutic effect, thus increasing the risk of exposure to toxicity or undesirable effects [26]. In addition, in other neoplastic diseases such as breast, melanoma, pancreatic and prostate cancers, bevacizumab failed to significantly increase patient survival, even when combined with chemotherapy.
BEYOND BEVACIZUMAB
Given the limitations of targeting VEGF with bevacizumab, other angiogenesis inhibitors were developed to target its key receptor, VEGFR-2 [27, 28].
Ramucirumab
Ramucirumab is a humanized IgG monoclonal antibody that targets the extracellular domain of VEGFR-2, thereby inhibiting angiogenesis by blocking VEGF binding to the receptor [27]. From preclinical studies it is evident that ramucirumab inhibits cell proliferation in vitro, as well as tumor progression in mouse xenograft models of human cancer [27]. The effects of the drug have also been studied in patients [28, 29].
In phase I clinical trials in patients with advanced neoplastic diseases such as NSCLC, gastric and colorectal cancers. Results showed that weekly and fortnightly schedules of ramucirumab were well tolerated, with the most common adverse effect being hypertension [29]. Another study was undertaken to evaluate ramucirumab as a second-line treatment in advanced gastric cancer.
The drug decreased the risk of disease progression by 37–52% and death by 19–22% [28]. Studies using dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) revealed that ramucirumab decreased tumor vascularity in 69% of treated patients, including those with multiple tumors [29].
Ramucirumab has also been shown to improve overall survival by an average of 5.2 months in a phase III clinical trial in patients with advanced gastric cancer [30]. In another Phase III trial ramucirumab was evaluated as single therapy and in combination with paclitaxel in patients with advanced gastric cancer or gastro-esophageal adenocarcinoma. The median OS was 5.2 months in the ramucirumab group, compared to 3.8 months in the placebo group [31].
Progression free survival was prolonged with a median of 2.1 months in the ramucirumab group vs. 1.3 months in the placebo group [31]. The overall survival with ramucirumab plus paclitaxel was 9.6 months vs 7.4 months with placebo plus ramucirumab [31].
Phase II trials evaluated ramucirumab in different combinations and in several neoplasms, with dacarbazine in melanoma, mitoxantrone/prednisone in prostate cancer, carboplatin/paclitaxel in NSCLC and with oxaliplatin/folinic acid/5-fluorouracil in colorectal cancer [28]. While ramucirumab has shown promising results, he outcome of these clinical trials will shed further light on the effectiveness of the drug when used in combination therapies.
Apatinib
Apatinib is another novel tyrosine kinase inhibitor that targets vascular endothelial growth factor receptor-2 [32]. In vitro apatinib was shown to decrease endothelial cell proliferation, migration and tube formation [32]. In preclinical studies using nude mice injected with lung or colon cancer cells, apatinib inhibited tumor progression [32]. A phase II clinical trial demonstrated the survival benefit of apatinib monotherapy in advanced NSCLC [33, 34]. The 12-month OS rate in the study was approximately 57% [32-34].
Another study revealed that apatinib monotherapy and apatinib plus docetaxel have potential as therapeutic options for heavily pretreated patients with advanced non-squamous NSCLC [35]. The study showed that both apatinib monotherapy and apatinib plus docetaxel treatment had a positive response, with progression-free survival durations of 5 months and 6 months respectively being attained [35]. In a different study Apatinib was well tolerated when administered as third-line or beyond therapy in patients with chemotherapy-refractory advanced or metastatic adenocarcinoma of the stomach or gastroesophageal junction. The main side-effect was hypertension [33].
Of interest is whether VEGFR-2 inhibitors are more effective than anti-VEGF therapy in inhibiting tumor angiogenesis and suppressing tumor progression. Very few studies have been conducted to compare these two approaches. It appears that different approaches have therapeutic benefits in different disease settings.
Effectiveness of Anti-VEGF Versus Anti-VEGFR Therapy
A study was undertaken to compare the efficacy of bevacizumab with cediranib, a VEGFR inhibitor, in a phase III study of advanced colorectal cancer [22]. The drugs were combined with oxaliplatin-based chemotherapy [22]. No significant difference in PFS was observed between the two regimens [22]. Although there were therapeutic benefits observed with the use of both VEGF and VEGFR-2 targeting therapies, such benefits have been modest.
DEVELOPMENT OF RESISTANCE TO VEGF/VEGFR-2 TARGETING DRUGS
The limitations of anti-VEGF therapy have been partly attributed to the triggering of alternative proangiogenic pathways [22, 26]. Although VEGF is a specific endothelial cell mitogen associated with angiogenesis, several other proangiogenic molecules can contribute to tumor angiogenesis, albeit not with the same potency [26].
Clinical trials have revealed that in some tumors blocking VEGF/VEGFR signaling can aggravate tumor hypoxia, which results in the tumor cells secreting proteins such as placental growth factor (PlGF), basic fibroblast growth factor (bFGF) and platelet derived growth factors (PDGFs) [26]. The latter were shown to stimulate angiogenesis and thus improve the supply of oxygen and nutrients to the tumor [26]. Other proangiogenic proteins that promote tumor angiogenesis and that seem to promote resistance to anti-VEGF therapy include basic fibroblast growth factor and angiopoietins [26]. These factors have also been linked to tumor aggressive growth in various neoplastic diseases such as melanoma, NSCLC and breast cancer [11, 36].
Platelet-Derived Growth Factor Pathway
Platelet-derived growth factors (PDGFs) are a family of peptides made up of PDGF-A, -B, -C, and -D that interact with trans-membrane tyrosine kinase receptors, PDGFR-α and -β [37]. The activation of PDGFs promotes angiogenesis by activating MEK/ Extracellular Signal-Regulated Kinase
(ERK), which leads to EC proliferation and migration (Fig. 3) [36]. Signaling through PDGFs also promotes the recruitment of pericytes and supports vessel maturation [36].
Genes associated with PDGF receptors are mutated in a number of malignancies. For instance, point mutations in PDGFR-α were found in 5% of gastrointestinal stromal tumors [37]. In addition, 5-10% of glioblastoma multiforme patients had an amplified PDGFR-α gene [37]. As a result of this gene amplification, the ECs in the tumor stroma become susceptible to stimulation by low levels of PDGF [37]. Such stimulation promotes tumor angiogenesis.
Fig. (3)) A diagram showing ligands that promote tumor angiogenesis and the pathways they elicit. Angiopoietin-2 (Ang-2) binds to the Tie-2 receptor and activates focal adhesion kinase (FAK), leading to vessel destabilization, while bFGF and VEGF bind FGFR and VEGFR-2 respectively to activate signaling through Src and nitric oxide synthase (NOS), which in turn promotes vessel permeability, EC proliferation and migration, and tube formation. Binding of VEGF to VEGFR-2 also promotes EC survival, through the PI3k, VEGF also promotes EC migration via FAK. Platelet-derived growth factor (PDGF) binds PDGFR and promotes angiogenesis through the MEK/ERK pathway. The AIs Pazopanib and Nintendab inhibit VEGFR-2, FGFR and PDGFR, while vanucizumab inhibits Ang-2 and VEGF.Due to the role of PDGFs and their receptors in human malignancies, a number of therapeutic molecules have been developed to target the PDGFRs [37]. The drugs include sorafenib and sunitinib, which do not only inhibit PDGFRs but also target other receptors involved in angiogenesis, such as fibroblast growth factor receptors [37].
Fibroblast Growth Factor Signaling in Anti-Cancer Therapy
Fibroblast growth factors (FGFs) are a family of pleiotropic ligands comprising of approximately 22 members that vary in size ranging from 17 kDa to 34 kDa [38]. The ligands bind to specific tyrosine kinase receptors known as fibroblast growth factor receptors (FGFRs) [39].
The overexpression, co-activation or mutation of these receptors contributes to cancer [39, 40]. For instance, the amplification of FGFR-1 is approximately 17% of non-squamous lung carcinoma patients, and approximately 15% of estrogen receptor positive breast cancer patients [40]. As a result, drugs that target FGFR-1 were developed [40]. However, the efficacy of FGFR-1 inhibition has yielded poor results and some of these inhibitors have not progressed beyond phase II trials [40].
Genetic alterations in FGFR-1 and FGFR-2, as well as in FGFR-3, -4 and -19, have been linked to cancer progression in preclinical models [41]. In addition, FGFR-2 is mutated in endometrial, lung and gastric cancers [42].
Drugs that target additional fibroblast growth factor receptors have been developed as tyrosine kinase receptor inhibitors and monoclonal antibodies [42]. The drugs exert their action by inhibiting FGFR dimerization or by preventing receptor phosphorylation [40]. Several of these drugs that target the FGF receptors, such as ponatinib and AZD4547, have progressed to clinical trials. The drugs may hold promise in the inhibition of tumor angiogenesis, especially in combination with other anti-cancer drugs.
Angiopoietin-Tie Pathway in Therapeutic Anti-Angiogenesis
Angiopoietins are a family of growth factors made up of angiopoietin (Ang)-1, Ang-2, Ang-3 and Ang-4. These growth factors bind to tyrosine kinase receptors Tie-1 and Tie-2, which are mainly expressed on endothelial cells (ECs) [12, 43]. Whereas Ang-1 is important for vessel maturation, Ang-2 induces vessel regression and is required during the early stages of angiogenesis [43]. The binding of Ang-1 to the Tie-2 receptor leads to the recruitment of pericytes to premature segments of newly formed vessels, thus promoting the re-enforcement of these vessels and their maturation [44, 45]. However, it is Ang-2 and the Tie-2 receptor that appear to play an important role in tumor angiogenesis. Indeed, some tumor cells express Ang-2 and the Tie-2 receptor [44].
The interaction of angiopoietin-2 with the Tie-2 receptor promotes tumor cell plasticity as well as the remodeling of the tumor vasculature [44]. Furthermore, the overexpression of Ang-2 has been linked to poor clinical prognosis in various cancers [46]. As a result, the design of drugs that target the tumor vasculature has also focused on targeting this pathway.
Vanucizumab was developed to target Ang-2/Tie-2 signaling. The drug is a bi-specific monoclonal antibody against Ang-2 and VEGF. A phase I study evaluating the safety, pharmacokinetics, pharmacodynamics and antitumor activity of vanucizumab in adults with advanced solid tumors (including renal cell and colon cancers) refractory to standard therapies was published recently [46].
The study found that bi-weekly doses of vanucizumab had an acceptable safety and tolerability profile consistent with single-agent use of selective inhibitors of the VEGF pathway [46].
Another study, a double-blind, randomized phase II study of vanucizumab (VAN) plus FOLFOX vs. bevacizumab (BEV) plus FOLFOX in patients with previously untreated metastatic colorectal carcinoma (mCRC) showed that VAN plus FOLFOX did not improve PFS compared to BEV plus FOLFOX [47]. Furthermore, the VAN-FOLFOX combination increased the risk of hypertension [47]. More studies are needed to identify effective doses and combination approaches using VAN with other drugs.
Various other AIs target receptors such as the epidermal growth factor receptor (EGFR) as well as adhesion molecules such as integrins (Table 1) may have potential in combination strategies and thus warrant further investigation.
The limitations of single molecule targeting AIs (Table 1) have necessitated a radical shift in the rationale for drug targeting. Indeed, the development of AIs that target more than one molecule or pathway simultaneously has become the focus in the quest to design more effective therapeutics.
THERAPEUTIC TARGETING OF MULTIPLE ANGIOGENIC PATHWAYS
Angiogenesis inhibitors that target several angiogenic pathways have been demonstrated to increase progression free survival in various tumors [28, 48-51]. These include multi-targeting tyrosine kinase inhibitors and monoclonal antibodies (Fig. 4) [28]. Some of the approved drugs that target multiple pathways in therapeutic angiogenesis are listed in Table 2.
Multi-Targeting Tyrosine Kinase Inhibitors
Tyrosine kinase inhibitors (TKIs) are small molecules that inhibit tyrosine kinases, a group of transmembrane receptors involved in various signaling pathways [16]. The kinases exert their effects through different mechanisms: i. they can compete with adenosine triphosphate (ATP), ii. they can catalyze the phosphorylation of other signaling molecules, iii. they can bind to a molecule allosterically at a site different from the active site, and thus alter the activity of that molecule [16, 52]. Anti-angiogenic TKIs have been employed in cancer treatment as single agents and in combination with other drugs [52]. The receptors targeted by these TKIs include VEGFRs, PDGFRs, EGFRs, c-KIT, fms-like tyrosine kinase 3 (FLT3) and FGFRs (Table 3) [52-54].
Some multi-targeting TKIs such as sorafenib and Sunitinib have been approved for the treatment of various neoplasms.
Fig. (4)) A diagram representing some of the key milestones in the development of angiogenesis inhibitors.Sorafenib
Sorafenib is a multi-kinase inhibitor that targets VEGFR-2, VEGFR-3, PDGFR and Raf [55, 56]. In vitro studies have revealed that sorafenib inhibits the proliferation of human hepatocellular carcinoma cells [56, 57]. In preclinical xenograft models of human hepatocellular carcinoma, the drug was shown to inhibit angiogenesis and tumor growth [57, 58].
Sorafenib has been employed in the treatment of various cancers as monotherapy and in combination with chemotherapy to improve patient outcome [59-61]. Hwang and colleagues (2017) observed complete remission following the administration of a combination of sorafenib and tegafur, an orally administered fluoro-uracil based chemotherapeutic, in a hepatocellular carcinoma (HCC) patient with progressed disease [61]. Sorafenib was also shown to improve OS in RCC patients [61-63].
In a multi-center phase III trial using sorafenib in advanced renal cell carcinoma (RCC), the 1-year PFS and OS were 58.4% and 64.6% respectively [63].
In some countries including those in the European Union and the USA, oral sorafenib is the approved treatment for patients with advanced HCC [64]. The various clinical trials have shown that sorafenib improves OS, especially when administered as part of combination therapies.
Sunitinib
Sunitinib, also known as Sunitinib malate, is a small-molecule inhibitor which targets vascular endothelial growth factor receptors (VEGFR-1, -2, -3), PDGFR-β, Flt3, glial cell-line derived neurotropic factor receptor (RET), macrophage colony stimulating factor 1 receptor (CSF-1R) and stem cell factor receptor (KIT) [65-68]. Sunitinib has demonstrated anti-angiogenic effects in in vitro models and anti-tumor effects in cancer cell lines that express its target receptors [66, 67].
The drug is approved in various countries for the treatment of advanced renal cell carcinoma as well as for imatinib resistant gastrointestinal stromal tumor (GIST) [68]. The mechanism of sunitinib in RCC has been attributed to its anti-angiogenic effects. Indeed, decreased levels of soluble VEGFR-2 were measured in plasma samples of RCC patients that were on sunitinib treatment [68]. In addition, in RCC and GIST patients, sunitinib induced a decrease in soluble KIT, a stem cell factor receptor that is involved in the promotion of angiogenesis [68-70].
Sunitinib has since become the reference standard of care for first line mRCC treatment [70]. In a global expanded clinical trial undertaken in 50 countries, sunitinib increased OS by 18.7 months, and clinical benefit was observed in naïve and previously treated patients [70]. Furthermore, clinical benefit was observed across age and even in patients with poor prognosis, including those with brain metastasis [70].
In treatment combinations employing sunitinib plus the chemotherapeutic drug gemcitabine in patients with sarcomatoid mRCC, and with sunitinib plus the immunotherapy drug AGS-003, there was improved survival in both instances and the therapies were well tolerated [69, 70]. It is anticipated that these treatments will improve the standard of care for RCC patients [70].
Axitinib
Other multi-targeting TKIs that have received approval for the treatment of various cancers (Table 2) include axitinib and pazopanib [55]. Axitinib is a second-generation inhibitor of VEGFR-1, VEGFR-2, and VEGFR-3. Axitinib inhibited tumor growth in a pre-clinical model of breast cancer. The drug also induced partial response in clinical trials involving patients with various cancers [55]. Axitinib is currently approved for the treatment of advanced renal-cell carcinoma [55].
Pazopanib
Pazopanib is another second generation TKI which inhibits VEGF receptors [73]. It inhibits the VEGF signaling pathway via ATP-competitive inhibition of VEGFRs, with IC50 values of approximately 10 nm, 30 nm, and 47 nm for VEGFR-1, VEGFR-2, and VEGFR-3, respectively [74]. The drug also inhibits PDGFR-α, PDGFR-β, fibroblast growth factor receptor (FGFR)-1, FGFR-3, and c-KIT [74]. Pazopanib has been shown to be effective in the treatment of patients with metastatic renal cell cancer [75].
Nintedanib
A recently developed TKI, nintedanib, was approved for the treatment of pulmonary fibrosis in 2014. The drug was later tested in a phase III clinical trial in non-small-cell lung carcinoma patients [76]. Nintedanib was evaluated in combination with docetaxel as second-line treatment, and the combination reduced tumor size by approximately 9 mm after 6 months [76].
Multi-Targeting Versus Mono-Targeting AIs
Few studies have been conducted to compare the efficacy of mono- and multi-targeting AIs in cancer treatment. A phase III study compared bevacizumab and sunitinib in breast cancer patients where both drugs were combined with paclitaxel [77].
Bevacizumab showed superior results as the median PFS was 7.4 months with sunitinib-paclitaxel vs. 9.2 months with bevacizumab-paclitaxel [77]. However, a systematic review of several clinical trials compared the clinical effectiveness of the two drugs in RCC. Evidence revealed that Sunitinib was statistically superior to bevacizumab plus interferon [78].
In addition to TKIs other molecules that target angiogenesis have been explored in various studies, especially for use in combination approaches [79].
FUSION PEPTIDES AND DECOY RECEPTORS
Other AIs that do not target RTKs, such as fusion peptides (aflibercept) and decoy receptors (abexinostat) have also been developed to target multiple proangiogenic pathways simultaneously.
Aflibercept
Aflibercept is a fusion protein that consists of portions of VEGFR-1 and -2 fused with the Fc portion of human IgG [80]. Aflibercept binds multiple proangiogenic factors (VEGF, VEGF-B and PlGF) and leads to their neutralization [80].
Preclinical studies in a xenograft model of colon cancer showed that aflibercept inhibits angiogenesis [80]. In clinical studies aflibercept was shown to improve PFS and OS in advanced colorectal cancer [80]. Furthermore, the drug was found to improve PFS in advanced non- small cell lung cancer, although it did not improve OS [81].
Phase III clinical studies in mCRC revealed that a combination of the drug with folinic acid, fluorouracil, and irinotecan (FOLFIRI), improved overall survival by 13.5 months [82]. Noteworthy is that the adverse effects with aflibercept, although similar to those observed with bevacizumab, were less severe.
Abexinostat and Panobinostat Attenuate Hypoxia
Abexinostat is a non-selective pan-histone deacetylase (HDAC) inhibitor that is in the experimental stage [83]. Of note is that HDAC has been associated with HIF-dependent gene expression and the promotion of hypoxia [83, 84]. Abexinostat thus suppresses hypoxia by inhibiting HDAC [83]. In pre-clinical studies the drug was found to be effective in inhibiting the growth of different types of tumors by inhibiting angiogenesis [85].
Previous studies have shown that hypoxia-induced alterations in the tumor microenvironment can promote resistance to AIs [86]. These alterations include the increased secretion of proangiogenic growth factors, mobilization of bone marrow-derived endothelial cells, the induction of epithelial-to-mesenchymal transition (EMT) and vessel co-option [87, 88].
Phase I clinical studies have revealed that abexinostat enhances the effects of the AI pazopanib [84]. In another study, the effect of a combination of pazopanib and abexinostat in cancer treatment was evaluated. Both drugs target hypoxia-inducible factor and VEGF-A respectively. With the inclusion of abexinostat, prolonged exposure of patients to pazopanib was possible and resistance was overcome [84]. Indeed, resistance is known to develop when pazopanib is used alone [89].
Another HDAC inhibitor, panobinostat, was tested in a phase II study in combination with rituximab, a monoclonal antibody against the B-lymphocyte antigen CD20 in 18 patients with relapsed diffuse large B-cell lymphoma [90]. The results of the study showed that the combination of panobinostat with rituximab induced a response in 11% of the patients [90
